A variety of different compositions for use as protective and lubricating coatings for titanium and its alloys have been proposed. Most of them are based on silica and include such additives as boric oxide, alumina, zinc oxide, lead oxide and phosphorus pentoxide. For example, British Pat. No. 921 861 discloses silica (SiO.sub.2) based glass compositions which also include boric oxide (B.sub.2 O.sub.3), at least one of the alkaline earth metal oxides MgO, CaO and BaO, and either or both of the alkali metal oxides Na.sub.2 O and K.sub.2 O, all in correlated ranges. It is observed in the description that a small quantity of alumina (Al.sub.2 O.sub.3) may be added to the composition to increase its viscosity. A glass comprising 37 mol % SiO.sub.2, 24 mol % B.sub.2 O.sub.3, 9 mol % BaO, 28 mol % K.sub.2 O and 2 mol % Al.sub.2 O.sub.3 is given as an example.
It has however been found that these silicate-based (in molar terms) glasses tend to be rather soft at the temperatures at which titanium and its alloys are worked--typically in the range from 750.degree.to 1050.degree. C. depending upon the alloy composition. In addition, with the exception of the potentially toxic and therefore undesirable lead silicate based glasses, their surface tension tends to be high and therefore they may not easily wet the metal surface.